The amphiphilic triblock copolymer poly(ethylene glycol)-co-polylactide-co-poly(ε-caprolactone) (PEG-PLA-PCL) with increasing PLA block ratio and corresponding diblock copolymer poly(ethylene glycol)-co-poly(ε-caprolactone) （PEG-PCL）were prepared using ring-opening polymerization of d,l- lactide and ε- caprolactone initiated by monomethoxy poly(ethylene glycol)(Mn=1900, 5000). The structure of the target copolymers was confirmed by means of FT-IR,1H-NMR spectra and GPC, then emulsifying properties of the copolymers and low molecular surfactants were studied and their relationship with the block ratio of the intermediate polar block PLA in triblock copolymers was discussed in detail. The results indicate that all of the copolymers can emulsify the toluene/water system and the emulsions were oil-in-water type. The copolymers show better emulsifying properties than the low molecular surfactants, and the emulsifying ability of the copolymers first increased and then decreased with the increase of PLA block ratio. It is because that the interactions with water and oil phase are enhanced by inserting intermediate polar block PLA, leading to a thicker interfacial membrane. To achieve optimum emulsifying performance, longer intermediate polar block was required for PEG1900-containing copolymers than that for PEG5000-containing copolymers.